1. Field of the Invention
The present invention relates to a power generation system having a constant pressure once-through boiler.
This application is based on Japanese Patent Application No. 2007-091784, the content of which is incorporated herein by reference.
2. Description of Related Art
In thermal electric power plants, power generation systems that have a constant pressure once-through boiler and a steam turbine as the main constituents are often employed (See for example, Japanese Unexamined Patent Application, Publication No. Hei 9-96227). Such power generation systems generate electric power by means of a steam turbine using steam generated by a constant pressure once-through boiler.
FIG. 3 shows an example of a conventional power generation system.
First, the steam for driving a steam turbine 34 is generated by a furnace 32. The steam generated by the furnace 32 flows through a first steam piping 31 provided with a boiler throttle valve 35, or a first steam throttle bypass piping 36 provided with a boiler throttle bypass valve 30, and is guided to a first superheater 37a. The steam that has been superheated by the first superheater 37a flows through a desuperheater 39 and is guided to a second superheater 37b. The steam that has been superheated again by the second superheater 37b flows through a second steam piping 33 and is then guided to the steam turbine 34. At this time, a turbine governing valve 38 provided in the second steam piping 33 adjusts a flow amount of the steam according to the load of the steam turbine 34.
In the conventional power generation system, after the pressure of a fluid (water vapor) is raised to a supercritical pressure at the furnace 32, this pressure is maintained up to the entry of the turbine governing valve 38, and the aperture of the turbine governing valve 38 is adjusted according to the load of the steam turbine 34.
Here, the electric power generated by the steam turbine 34 is known to be approximately proportional to the product of the pressure of the steam flowing in, and the aperture of the turbine governing valve 38. As described above, since the steam pressure in the second steam piping 33 is constant (supercritical pressure) up to the inlet of the turbine governing valve 38, in the case where the electric power required to be generated by the steam turbine 34 is low, the aperture of the turbine governing valve 38 needs to be significantly narrowed. As a result, there has been a problem in that the efficiency of the steam turbine 34 is reduced due to a loss in steam pressure at the turbine governing valve 38.
Moreover, if the throttle ratio at the turbine governing valve 38 becomes greater, the steam temperature drop at the inlet of steam turbine 34 also becomes greater due to a significant influence of adiabatic expansion. However, since the fluctuation ratio of the steam turbine 34 inlet temperature that can be handled by the steam turbine 34 is limited, also the fluctuation ratio of the aperture of the turbine governing valve 38 naturally becomes limited. As a result, there has been a problem in that the steam turbine 34 cannot be operated so as to comply with the required electric power.